1. Technical Field Text
The present invention relates to an implantable medical device, such as a stent graft. In the preferred embodiment, the device includes a plurality of bridging elements coupled at a proximal and/or distal end of the medical device.
2. Background
In the medical field it is now commonplace to use stent grafts and other implantable medical devices to treat a variety of medical conditions such as aneurysms, dissections, occlusions and the like. Stent grafts are generally formed of a tubular member of graft material (which may or may not be bifurcated or have side branches) and one or more stents provided on the graft member to retain this in an open position. The stents may be located on the inside or the outside of the graft member, or both, as desired in connection with the particular medical device or medical application. Some vena cava filters are also in the form of a stent graft, that is have a graft element which is supported by a stent structure. The teachings herein apply equally to such vena cava filters.
Stents used in stent grafts come in a variety of forms, the most common of which is a stent ring formed of a series of strut elements arranged in a zig-zag or sinusoidal manner. The advantage of a zig-zag or sinusoidal stent is that this can be readily compressed onto an introducer for endoluminal deployment of the stent graft. Furthermore, this arrangement of the strut elements gives the stent a good expansion strength, that is the enable the stent to expand and press against a vessel wall in so doing to support the graft member in an open configuration and to hold the medical device in position.
Zig-zag stents of such a nature can be considered to be formed of a plurality of stent struts which are coupled to one another at their ends to provide a structure having a plurality of peaks and valleys at the junctions between adjacent stent struts.
Greater compressibility and flexibility of the medical device can be achieved with a zig-zag structure which is more open (that is, where the angle between adjacent and connected stent struts is relatively greater). This is particularly useful for stent grafts which need to be compressed to a very small diameter for introduction and implantation into a patient, as well as in cases where the stent graft needs to remain flexible as a result of the physiology of the lumen into which the stent graft is to be implanted.
There is typically provided an end stent located at the proximal end of the stent graft (that is the end which is located upstream or closer to the heart relative to the remainder of the stent graft), in order to keep this end open. There is commonly also provided a stent at least at the distal end of the stent graft and generally also one or more stents in intermediate positions along the graft member.
The proximal end of the stent graft needs to be retained fully open so as to ensure that this properly seals against the internal walls of the patient's vessel and must also be retained in the correct position in the vessel, both during and after deployment. Where the end most stent is of a zig-zag or sinusoidal structure, particularly a relatively open structure to increase the flexibility and compressibility of the stent graft, this can lead to unsupported flaps of graft material between adjacent peaks of the stent. These flaps of graft material will tend to be urged radially inwardly towards the centre of the lumen as a result of blood flow impinging upon the proximal end of the stent graft. This will result in an incomplete seal of the graft material to the inner vessel wall. When this endoleak, blood leakage occurs, there is the risk of: a) an endoleak, in which blood can flow passed the device and the lumen wall; b) migration of the stent graft with the patient's lumen; and c) also premature wear and tear of the stent graft caused by the flapping of the graft material.
It is known to locate at the proximal end of the graft member one or more bare stents which extend beyond the extremity of the graft member and which can support the proximal end of the graft member, particularly those portions not supported by the other stents of the stent graft to urge these portions against the vessel walls. Bare stents of this nature can also be provided with barbs which anchor into the vessel wall.
Although bare stents have particular uses in some medical applications, they are not suitable in a number of circumstances as they can cause erosion of the vessel wall. Bare stents are generally not ideal in very tortuous anatomies. They can in some circumstances also reduce the flexibility of the stent graft assembly.
Other forms of stent located at the proximal end of the stent graft and specifically designed to overcome the problem of support of its proximal end will impair the compressibility of the stent graft as well as, in some circumstances, its flexibility in situ.